In transferring data between a host central processor unit and a disk storage unit, the cost of the disk system, such as a "hard" disk system in contrast with a more flexible "cartridge" or "floppy" disk system, has been relatively high where high data densities and high transfer rates are required. Normally, such disks have been used with relatively expensive host central processor units in which the channel data transfer rate is higher than that of the disk transfer rate. In such situations there is usually no problem in transferring data directly to and from the disk system with reference to the host central processor unit.
However, as the cost of such disks have become considerably reduced, the use of such hard disks with high data density and high transfer rates is now possible with much less expensive host central processor units. In such systems, however, the host central processor unit channel data transfer rate is often lower than that of the disk system. Accordingly, data cannot be directly transferred to the host central processor unit since the latter transfer rate cannot keep up with the disk system transfer rate and direct transfer becomes substantially impossible. In order to avoid such a problem such systems often use large capacity "first in-first out (FIFO)" buffer storage systems. In accordance therewith each data word obtained from the disk during a read operation, for example, is supplied to the buffer and temporarily stored before it is transferred to the host central processor unit. Because of the high data transfer rate of the disk relative to the channel transfer rate to the CPU, more and more data words remain stored in the FIFO buffer before they are transferred out to the CPU. If a large number of data words are required to be transferred, a relatively large capacity FIFO buffer is required and the expense of the overall transfer system can become relatively high.
Other techniques utilizing various complex systems of multiplexer units have also been utilized for such purpose and the costs thereof are also relatively high, particularly where the disk system is to be used with a relatively inexpensive processor. Therefore, it is desirable to design a system which will provide a low cost technique for transferring data between a central processor unit and a hard disk storage unit wherein the data transfer rate of the latter unit is substantially faster than the channel data transfer rate with respect to the central processor unit.